The Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and moderating temperature extremes between day and night.
The atmosphere has no abrupt cut-off. It slowly becomes thinner and thinner until it fades away into space. As such there is no definite boundary between the atmosphere and outer space, although an altitude of 120 km (75 mi or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. Additionally the Karman line, at 100 km (62 mi), is also frequently used as the boundary between atmosphere and space.
Compared to the radius of the Earth, its atmosphere is very thin. 99% of the mass of the atmosphere lies below 30 km, or 0.5% of the equatorial radius.
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